Voltage Variable Capacitors (VVCs) such as varactor diodes or voltage tunable dielectric capacitors are devices whose capacitance changes as a function of the applied voltage. This makes them useful in circuits such as voltage-controlled oscillators or voltage controlled matching networks where a tuning or bias voltage is applied to bring the VVC to the capacitance required for the circuit to work with a signal voltage, typically a modulated AC signal.
When the amplitude of the AC signal is not much less than the bias voltage, it itself affects the capacitance as it goes through a cycle. The capacitance is no longer constant and so a perfect sine wave applied to the capacitance is no longer a perfect sine wave but contains harmonics. In the presence of more than one sine wave, there are also mixing products while a modulated signal will experience spectral regrowth. Harmonics will be generated in the circuit as shown in FIG. 1, shown generally as 100, where VS 120 is a perfect sinusoidal voltage source and VB 110 is the applied bias voltage.
Rw 130, the impedance of the voltage source, is of the same order of magnitude as the impedance of the VVC at the signal frequency, i.e., 1/(ω·C), where ω is the angular frequency (2×π×frequency in Hz) of the signal voltage and C is the capacitance of the VVC. The DC feed is a means of applying the DC bias voltage VB 110 to the VVC so that it attains the desired capacitance without providing a connection to ground for the signal voltage and thus affecting the AC characteristics of the circuit. The DC feed could be implemented in a number of ways, such as with a resistor Rb that is much greater than 1/(ω·C) but much less than the DC leakage resistance of the VVC, or with an inductor of a value L such that its impedance ω·L is much greater than 1/(ω·C).
Thus, there is a strong need for an invention that would eliminate this non-linear behavior and make the capacitance of the VVC dependent on the bias voltage VB 110 but independent of the AC voltage VS 120.